Electrophotographic marking is a well-known method of copying or printing documents. Electrophotographic marking is performed by first exposing a substantially uniformly charged photoreceptor with a light image representation of a desired document. In response to that light image the photoreceptor discharges, creating an electrostatic latent image of the desired document on the photoreceptor's surface. Toner particles are then deposited onto that latent image, forming a toner image. That toner image is then transferred from the photoreceptor onto a substrate such as a sheet of paper. The transferred toner image is then fused to the substrate, usually using heat and/or pressure, thereby creating a copy of the desired image. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the production of another image.
The foregoing broadly describes black and white electrophotographic marking. Electrophotographic marking can also produce color images by repeating the above process for each color of toner that is used to make the composite color image. For example, in one color process, referred to as the REaD IOI process (Recharge, Expose, and Develop, Image On Image), a charged photoreceptor is exposed to a light image which represents a first color, say black. The resulting electrostatic latent image is then developed with black toner particles to produce a black toner image. A recharge, expose, and develop process is repeated for a second color, say yellow, then for a third color, say magenta, and finally for a fourth color, say cyan. The resulting composite color image is then transferred and fused onto a substrate.
One way of exposing a photoreceptor is to use a light emitting diode based exposure station. Such exposure stations are generally comprised of an elongated array of discrete light emitting diodes (LEDs) and an array of gradient index lenses that focus the light from the light emitting diodes onto the photoreceptor. To achieve high resolution (usually measured in spots per inch, or SPI), a large number of light emitting diodes are included in the LED array. In practice, each LED images a small area, referred to as a pixel, of the electrostatic image. By selectively driving the LEDs according to video data information a desired electrostatic line image comprised of a large number of individual pixels is produced on the photoreceptor. Since the photoreceptor moves relative to the light emitting diode based exposure station, by exposing the photoreceptor linewise a desired final image can be produced.
In light emitting diode based exposure stations the gradient index lens array is positioned between the light emitting diode array and the surface of the photoreceptor. Gradient index lens arrays, such as those produced under the trade name "SELFOC" (a registered trademark in Japan that is owned by Nippon Sheet Glass Company, Ltd.) are comprising of bundled gradient index optical fibers, or rods, reference U.S. Pat. No. 3,658,407. That patent describes a light conducting rod made of glass or synthetic resin which has a cross-sectional refractive index distribution that varies parabolically outward from the center of the rod. Each rod acts as a focusing lens for light introduced at one end. Relevant optical characteristics of gradient index lens arrays are described in an article entitled "Optical properties of GRIN fiber lens arrays: dependence on fiber length", by William Lama, Applied Optics, Aug. 1, 1982, Vol 21, No. 15, pages 2739-2746. That article is hereby incorporated by reference.
While light emitting diode based exposure stations are generally successful, their use is not without problems. One set of problems relates to their physical size in the process direction. Waterfront is a term for the process direction photoreceptor space that is taken up by a processing station. While light emitting diode arrays and gradient index lenses arrays themselves tend to be narrow, the required physical mounting, electrical drives, and cooling assemblies tend to be wide, in the order of 75 millimeters or so. This creates a problem when attempting to use light emitting diode based exposure stations. Simply put, waterfront is at a premium. A charging system, multiple exposure stations, multiple developers, a transfer station, and a cleaning station all must be located adjacent the photoreceptor. When designing an electrophotographic marking machine with the light emitting diode based exposure stations the waterfront requirements of the exposure stations directly impact cost.
A way of increasing the waterfront is to use a longer conjugate lens. However, long conjugate lenses are typically less radiometrically efficient or have lower resolution.
Therefore, radiometrically efficient light emitting diode based exposure stations having reduced waterfront requirements would be beneficial. Even more beneficial would be electrophotographic marking machines that use light emitting diode based exposure stations having a reduced waterfront requirement.